High purity tin and method for manufacturing same

ABSTRACT

Provided is a high purity tin (Sn) having an extremely low oxygen content. A high purity tin having a tin purity of 5N (99.999% by mass, provided that carbon, nitrogen, oxygen and hydrogen are excluded) or more, wherein the high purity tin has an oxygen content of less than 10 ppb by mass, as measured by elemental analysis using Dynamic-SIMS.

TECHNICAL FIELD

The present invention relates to high purity tin (Sn) having extremelylow oxygen content.

BACKGROUND ART

Commercially available high purity tin is generally produced by anelectrolytic method using an acidic tin solution such as tin sulfamate,tin sulfate, tin chloride or the like.

For example, Japanese Patent Application Laid-open Publication No.S62-1478 B (Patent Literature 1) describes a method for carrying outelectrolysis in an electrolytic bath having a liquid compositioncontaining 30 to 150 g/L of Sn and 30 to 200 g/L of sulfamic acid thatcontains few radioisotope elements, using 99 to 95% by weight or more oftin as an anode, under electrolytic conditions of a cathode currentdensity of 0.5 to 2.0 Amp/dm² and a liquid temperature of 15 to 50° C.,for the purpose of lowering a emission (claim 2 of Patent Literature 1).

Japanese Patent No. 2754030 B1 (Patent Literature 2) describes a methodfor producing tin characterized in carrying out electrolysis in anelectrolytic solution containing 90 to 240 g/L of sulfuric acid that atleast conforms to the standard of the first class grade sulfuric aciddefined in JIS K 8951 and 10 to 50 g/L of hydrochloric acid that atleast conforms to the standard of the first class grade hydrochloricacid defined in JIS K 8180 using tin having a purity of 99.97% by weightor more as an anode, for the purpose of lowering a emission (claim 1 ofPatent Literature 2).

Japanese Patent 3882608 B1 (Patent Literature 3) describes a method forremoving lead by electrolytic refining of impurities in metallic tin.More particularly, it describes a method of electrolytic refining ofhigh purity tin using an electrolytic solution made from a mixed acid ofsulfuric acid and silicofluoric acid, which includes withdrawing the tinelectrolytic solution from an electrolytic bath and routing it to aprecipitation tank; adding strontium carbonate to the electrolyticsolution in the precipitation tank to precipitate lead in the solutionat a liquid temperature of 35° C. or lower; then routing theelectrolytic solution containing said precipitates to a filter unit toseparate the precipitates by filtration; and recycling theprecipitates-removed electrolytic solution to the electrolytic bath andcarrying out electrolytic refining of tin (claim 1 of Patent Literature3).

Japanese Patent No. 5296269 B1 (Patent Literature 4) discloses a methodfor conducting electrolytic refining by leaching tin as a raw materialwith an acid such as sulfuric acid to form a leached solution as anelectrolytic solution, suspending an adsorbent for impurities in theelectrolytic solution, conducting the electrolytic refining using a rawmaterial Sn anode, thereby obtaining high purity tin having a purity of5 N or more (excluding gas components of 0, C, N, H, S and P). Moreparticularly, it describes a method for conducing electrolytic refiningin a sulfuric acid bath or a hydrochloric acid bath using tin having 3 Nlevel as the anode at an electrolysis temperature of 10 to 80° C. andunder a condition of a current density of 0.1 to 50 A/dm². It alsodiscloses that impurities are adsorbed by suspending an oxide such astitanium oxide, aluminum oxide and tin oxide, as well as activatedcarbon, and carbon in the electrolytic solution.

On the other hand, non-Patent Literature 1 discloses that tin is used asa raw material of a tin droplet target used for generating an EUV(Extreme Ultraviolet) light source for lithography.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Laid-open    Publication No. S62-1478 B2-   Patent Literature 2: Japanese Patent No. 2754030 B1-   Patent Literature 3: Japanese Patent No. 3882608 B1-   Patent Literature 4: Japanese Patent No. 5296269 B1

Non-Patent Literature

-   Takeshi Higashiguchi, Akira Endo and Kei Mizoguchi, “Current Status    of EUV Light Source Development”, Journal of Plasma and Fusion    Research, The Japan Society of Plasma Science and Nuclear Fusion    Research Vol. 89, No. 6, June 2013, p. 341-348

SUMMARY OF INVENTION Technical Problem

According to the manufacturing method disclosed in the prior art, highlypurified tin can be obtained. However, the present inventors have foundthat the use of tin obtained by the prior art as a raw material of a tintarget material employed for generating EUV light in the EUV (ExtremeUltraviolet) light source for lithography, is likely to lead to cloggingof nozzle of a droplet generator and disrupt the lithographic step whenthe tin target was heated in the EUV light source device and melted toform droplets having very small diameters of about several tenmicrometers.

The present inventors have studied that cause, and found that tinelectrolytically refined by a wet process using an electrolytic solutionsuch as sulfuric acid contains a large amount of oxygen and sulfur(including a case where they are mixed as oxides and sulfides), andthese contaminants adversely affect properties. According to theconventional wet process, metal impurities can be removed, but it isdifficult to remove oxygen and sulfur.

The present invention has been made in view of the above circumstances.One of the objects of the present invention is to provide high puritytin having a very low oxygen content, and preferably high purity tinhaving very low oxygen and sulfur contents. Another object of thepresent invention is to provide a method for manufacturing high puritytin having a very low oxygen content, and preferably high purity tinhaving very low oxygen and sulfur contents.

Solution to Problem

As a result of extensive studies to solve the above problems, thepresent inventors have found that when electrolytically refined tin isheated under vacuum in a state where a certain amount of carbon iscontained in the tin, oxygen in the form of free oxygen or oxide in thetin will react with carbon to form carbon monoxide or carbon dioxidewhich is removed by volatilization. Thus, the present inventions havefound that extremely low oxygen concentration in tin can be achieved.Further, the present inventors have also found that by such operation, asulfur content in tin is significantly reduced. Since sulfur has a highvapor pressure, it is removed as elementary sulfur by vacuum heating, aswell the sulfur is also removed as a compound such as sulfur oxide,which is considered to contribute to reduction in the amount of oxygenand the amount of sulfur. The present invention has been accomplishedbased on the above findings.

In one aspect, the present invention provides a high purity tin having atin purity of 5N (99.999% by mass, provided that carbon, nitrogen,oxygen and hydrogen are excluded) or more, wherein the high purity tinhas an oxygen content of less than 10 ppb by mass, as measured byelemental analysis using Dynamic-SIMS.

In one embodiment of the high purity tin according to the presentinvention, the high purity tin has a sulfur content of less than 0.1 ppmby mass, as measured by elemental analysis using GD-MS.

In another embodiment of the high purity tin according to the presentinvention, the sulfur content is less than 0.01 ppm by mass, as measuredby elemental analysis using GD-MS.

In yet another embodiment of the high purity tin according to thepresent invention, the high purity tin has a carbon content of less than10 ppb by mass, as measured by elemental analysis using Dynamic-SIMS.

In another aspect, the present invention provides a method formanufacturing a high purity tin, including subjecting tin having apurity of 3N (99.9% by mass, provided that carbon, nitrogen, oxygen andhydrogen are excluded) or more, and having an oxygen content of 5 ppm bymass or more and a carbon content of 10 ppm by mass or more as measuredby elemental analysis using a nondispersive infrared absorption method,to a vacuum heating treatment until the oxygen content in the tin isdecreased to be less than 10 ppb by mass as measured by elementalanalysis using Dynamic-SIMS.

In one embodiment of the method for manufacturing the high purity tinaccording to the present invention, the vacuum heating treatment iscarried out at a temperature of 400° C. or more and a degree of vacuumhigher than 1×10⁻³ Pa (absolute pressure).

In another embodiment of the method for manufacturing the high puritytin according to the present invention, the vacuum heating treatment iscarried out for 3 hours or more.

In yet another embodiment of the method for manufacturing the highpurity tin according to the present invention, further includingelectrolytically refining raw material tin using an electrolyticsolution containing at least one carbon-containing compound to obtainthe tin, wherein the raw material tin has a purity of 3N (99.9% by mass,provided that carbon, nitrogen, oxygen and hydrogen are excluded) ormore, and has an oxygen content of 5 ppm by mass or more and a carboncontent of less than 10 ppm by mass, as measured by elemental analysisusing a nondispersive infrared absorption method.

In yet another embodiment of the method for manufacturing the highpurity tin according to the present invention, the at least onecarbon-containing compound is a leveling agent.

In yet another embodiment of the method for manufacturing the highpurity tin according to the present invention, it further includesadding elemental carbon to a raw material tin to obtain the tin, whereinthe raw material tin has a purity of 3N (99.9% by mass, provided thatcarbon, nitrogen, oxygen and hydrogen are excluded) or more, and has anoxygen content of 5 ppm by mass or more and a carbon content of lessthan 10 ppm by mass, as measured by elemental analysis using anondispersive infrared absorption method.

In yet another embodiment of the method for manufacturing the highpurity tin according to the present invention, carbon atom is present ata ratio of 1 mole or more and 500 mole or less per 1 mole of oxygen atomin the tin.

In yet another embodiment of the method for manufacturing the highpurity tin according to the present invention, carbon atom is present ata ratio of 50 mole or more and 100 mole or less per 1 mole of oxygenatom in the tin.

In another aspect, the present invention provides a tin target materialmade of the high purity tin according to the present invention, used forgenerating EUV light in a lithographic EUV light source.

Advantageous Effects of Invention

According to an embodiment of the present invention, free oxygen andoxides present on the surface as well as in the inside can beeffectively reduced, so that it is possible to obtain high purity tinhaving an oxygen content of less than 10 ppb by mass, as measured byelemental analysis using secondary ion mass spectrometry (hereinafterreferred to as “Dynamic-Secondary Ion Mass Spectrometry: Dynamic-SIMS”).According to another embodiment of the present invention, it is possibleto obtain high purity tin having a carbon content of less than 10 ppb bymass, as measured by elemental analysis using Dynamic-SIMS. According tostill another embodiment of the present invention, it is possible toobtain high purity tin having a sulfur content of less than 0.01 ppm bymass, as measured by elemental analysis using glow discharge massspectrometry (hereinafter referred to as Glow Discharge MassSpectrometry: GD-MS).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process flow chart showing an example of a refining methodin the case of adding a carbon-containing compound during electrolysisof tin (Examples 1 to 4).

FIG. 2 is a process flow chart showing an example of a refining methodin the case of adding elemental carbon during vacuum heating treatment(Examples 5 to 8).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail below.In one embodiment, the method for manufacturing a high purity tinincludes subjecting tin having a purity of 3N (99.9% by mass) or more,and having an oxygen content of 5 ppm by mass or more and a carboncontent of 10 ppm by mass or more as measured by elemental analysisusing a nondispersive infrared absorption method, to a vacuum heatingtreatment such that the oxygen content in the tin is decreased to beless than 10 ppb by mass as measured by elemental analysis usingDynamic-SIMS. The tin purity described in the present invention is avalue excluding carbon, nitrogen, oxygen and hydrogen and can bemeasured by the GD-MS method. The measurements of impurity elementscontained in the high purity tin means results obtained by the analysisof targeted 73 impurity element components of Li, Be, B, F, Na, Mg, Al,Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As,Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sb, Te, I, Cs,Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W,Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th and U, when the tin is consideredto be a matrix.

As a result of mass spectrometric analysis by the GD-MS method, oneembodiment of the high purity tin according to the present inventionshows that each of Li, Be, B, F, Na, Mg, Al, Si, Cl, K, Ca, Sc, Ti, V,Cr, Mn, Co, Ni, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh,Pd, Ag, Cd, In, Te, I, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Th and U is lessthan a detection limit.

In the present invention, the expression “less than a detection limit”means that Sc and V are less than 0.001 ppm by mass; Li, Be, B, Ti, Cr,Mn, Fe, Cu, Ga, As, Rb, Sr, Y, Zr, Nb, Rh, Pd, Ag, Ce, Nd, Sm, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu, Bi, Th and U are less than 0.005 ppm by mass;Na, Mg, Al, Si, P, S, Cl, K, Ca, Co, Ni, Zn, Ge, Se, Mo, Ru, Eu, Hf, W,Re, Os, Ir, Pt and Pb are less than 0.01 ppm by mass; Tl is less than0.02 ppm by mass; F, Br, Cd, I, Cs, Au and Hg are less than 0.05 ppm bymass; Te, Ba, La and Pr are less than 0.1 ppm by mass; Sb is less than0.5 ppm by mass; In is less than 1 ppm by mass; and Ta is less than 5ppm by mass. The element concentration less than the detection limit isconsidered to be zero.

(A. Raw Material Tin)

A commercially available raw material tin having a purity at a level of2N (purity 99% by mass) contains a large number of impurities andcontaminants. For example, the raw material tin contains compounds inwhich oxygen or sulfur is bonded to tin, such as tin oxide (SnO, SnO₂)or tin sulfide (SnS, SnS₂), and contaminants (non-metallic inclusionssuch as SiO₂ from the outside of the system) on the surface and theinside. Further, the raw material tin contains a large amount ofcompounds of metals such as iron, lead, zinc and copper which will bethe contaminants in tin.

Therefore, considering that the use of tin having a lower purity gives aburden to a refining step, the raw material tin may preferably have apurity of 99.9% by mass (3N) or more, and more preferably have a purityof 99.995% by mass (4N5) or more with the exception of carbon, nitrogen,oxygen and hydrogen. However, if the raw material tin having excessivelyhigh purity is used, economic efficiency will be deteriorated.Therefore, the raw material tin may typically has a purity of from 99.9to 99.99% by mass (from 3N to 4N), and more typically a purity of from99.95 to 99.98% by mass (from 3N5 to 3N8). Such high purity raw materialtin may typically have an oxygen content of 5 ppm by mass or more, andmore typically from 10 to 50 ppm by mass, as measured by elementalanalysis using the nondispersive infrared absorption method, and maytypically have a sulfur content of 0.5 ppm by mass or more, and moretypically from 1 to 30 ppm by mass, as measured by elemental analysisusing the GD-MS method.

In the present invention, oxygen contained in the raw material tin isremoved using carbon. Therefore, tin before the vacuum heating treatmentwill need to contain carbon at a predetermined concentration. A suitablecarbon content may vary depending on the oxygen content, but the carbonatom may be preferably present at a ratio of one mole or more, and morepreferably 10 mole or more, and even more preferably 50 mole or more perone mole of oxygen atom, in terms of removal efficiency of oxygen.However, if the carbon content in tin before the vacuum heatingtreatment is too high, unreacted carbon will float on the surface ofmolten tin during the vacuum heating treatment and will be a waste.Therefore, the carbon atom may be preferably present at a ratio of 500mole or less, and more preferably 100 mole or less per one mole ofoxygen atom.

(B. Electrolytic Refining)

A commercially available Tin having a purity of 3N or more often has alower carbon concentration of less than 10 ppm by mass. In such a case,the carbon content in the tin can be increased so as to obtain aconcentration of from 10 to 100 ppm by mass, by subjecting the tinhaving a purity of 3N or more as a raw material to electrolyticrefining, adding elemental carbon to the resulting electrolyticallyrefined tin and carrying out the vacuum heating treatment as describedbelow, or subjecting the tin to electrolytic refining using anelectrolytic solution containing at least one carbon-containingcompound. The electrolytic refining can be carried out by electrolyticrefining using the raw material tin as an anode, or by electrowinningafter chemically leaching the raw material tin with sulfuric acid.

For sulfuric acid used in the sulfuric acid leaching, a pH of a tinsulfate solution after leaching may be preferably 1 or less in order toprevent generation of tetravalent tin.

The concentration of tin in the tin sulfate solution obtained by thesulfuric acid leaching may be preferably 30 g/L or more, and morepreferably 50 g/L or more, and still more preferably 70 g/L or more, inorder to increase productivity. Further, the concentration of tin may bepreferably 120 g/L or less, and more preferably 100 g/L or less, for thereason that precipitation does not occur.

The temperature of sulfuric acid used for the sulfuric acid leaching maybe preferably 100° C. or less, and more preferably 90° C. or less, andstill more preferably 80° C. or less, for safety reasons. On the otherhand, the temperature of sulfuric acid may be preferably 50° C. or more,and more preferably 60° C. or more, and still more preferably 70° C. ormore, in terms of reaction kinetics.

Tin may be electrodeposited on the cathode by electrowinning from asulfuric acid-acidic tin sulfate electrolytic solution obtained bysulfuric acid leaching. For the reason of reducing an oxide layer, theelectrodeposited tin may be preferably in the form of a plate. Thetemperature of the electrolytic solution during electrowinning may bepreferably 10° C. or more, and more preferably 20° C. or more, and evenmore preferably 30° C. or more, in order to decrease resistance of theelectrolytic solution (increasing electric conductivity). On the otherhand, the temperature may be preferably 80° C. or less, and morepreferably 60° C. or less, and still more preferably 40° C. or less, forsafety and economic reasons.

The current density during electrolytic refining may be preferably 0.1A/dm² or more, and more preferably 0.5 A/dm², and even more preferably1.0 A/dm² or more, in terms of productivity. Further, the currentdensity during electrolytic refining may be preferably 10 A/dm² or less,and more preferably 5 A/dm² or less, and even more preferably 3 A/dm² orless, for the reason of suppressing the generation of tetravalent tinand of countermeasures against heat generation.

When at least one carbon-containing compound is added to theelectrolytic solution, the concentration of the carbon-containingcompound to be added to the electrolytic solution may vary depending onthe content of carbon to be added to the tin, and the carbon-containingcompound may be preferably added such that the relationship between thecarbon concentration and the oxygen concentration in the tin is in therange as described above. As an example, the concentration of thecarbon-containing compound in the electrolytic solution may bepreferably from 0.01 to 100 g/L, and more preferably from 0.1 to 50 g/L,and even more preferably from 1 to 10 g/L.

The carbon-containing compound may be preferably a leveling agent. Sincetin that is smooth and has small surface area can be thus obtained, aneffect of suppressing generation of oxides on the surface can also beprovided. The leveling agent that can be used includes varioussurfactants such as anionic, cationic, nonionic or amphotericsurfactants. Among them, the nonionic surfactant can be preferably usedbecause it does not contain sodium, potassium or the like. Examples ofthe nonionic surfactant include polyoxyethylene alkyl ether-basedsurfactants, polyoxyethylene alkyl phenyl ether-based surfactants, andpolyoxyethylene alkyl amino ether-based surfactants.

Examples of the polyoxyethylene alkyl ether-based surfactants that canbe suitably used include polyoxyethylene decyl ether, polyoxyethylenelauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearylether, polyoxyethylene oleyl ether, polyoxyethylene coconut alcoholether, polyoxyethylene-2-ethylhexyl ether, polyoxyethylene syntheticalcohol ether, polyoxyethylene secondary alcohol ether, polyoxyethylenetridecyl ether and the like. Further, examples of the polyoxyethylenealkyl phenyl ether-based surfactants that can be suitably used includepolyoxyethylene octyl phenyl ether, polyoxyethylene nonyl phenyl ether,polyoxyethylene cumyl phenyl ether, polyoxyethylene polynuclear phenylether, polyoxyethylene-β-naphthol ether, polyoxyethylenebisphenol-A-ether, polyoxyethylene dodecyl phenyl ether, polyoxyethylenestyrenated phenyl ether, nonylphenol novolac EO adducts and the like.Further, examples of the polyoxyethylene alkyl amino ether-basedsurfactants that can be suitably used include polyoxyethylene laurylamino ether, polyoxyethylene beef tallow amino ether, polyoxyethyleneoleyl amino ether, polyoxyethylene stearyl amino ether, EO adducts ofN,N-bis(2-hydroxyethyl)-N-cyclohexylamine and the like.

Examples of other leveling agents include β-naphthol,β-naphthol-6-sulfonic acid, β-naphthalenesulfonic acid,m-chlorobenzaldehyde, p-nitrobenzaldehyde, p-hydroxybenzaldehyde, (o-,p-)methoxybenzaldehyde, vanillin, (2,4-, 2,6-)dichlorobenzaldehyde, (o-,p-)chlorobenzaldehyde, 1-naphthaldehyde, 2-naphthaldehyde,2(4)-hydroxy-1-naphthaldehyde, 2(4)-chloro-1-naphthaldehyde,2(3)-thiophenecarboxyaldehyde, 2(3)-furaldehyde,3-indolecarboxyaldehyde, salicylaldehyde, o-phthalaldehyde,formaldehyde, acetaldehyde, paraldehyde, butylaldehyde,isobutylaldehyde, propionaldehyde, n-valeraldehyde, acrolein,crotonaldehyde, glyoxal, aldol, succindialdehyde, capronaldehyde,isovaleraldehyde, allylaldehyde, glutaraldehyde,1-benzylidene-7-heptanal, 2,4-hexadienal, cinnamaldehyde,benzylcrotonaldehyde, amine-aldehyde condensates, mesityl oxide,isophorone, diacetyl, hexanedione-3,4, acetylacetone,3-chlorobenzylideneacetone, sub.pyridylidene acetone, sub.furfuridineacetone, sub.tenylidene acetone, 4-(1-naphthyl)-3-buten-2-one,4-(2-furyl)-3-buten-2-one, 4-(2-thiophenyl)-3-buten-2-one, curcumin,benzylidene acetylacetone, benzalacetone, acetophenone, (2,4-,3,4-)dichloroacetophenone, benzylidene acetophenone,2-cinnamylthiophene, 2-(w-benzoyl)vinylfuran, vinylphenylketone, acrylicacid, methacrylic acid, ethacrylic acid, ethyl acrylate, methylmethacrylate, butyl methacrylate, crotonic acid,propylene-1,3-dicarboxylic acids, cinnamic acid, (o-, m-, p-)toluidine,(o-, p-)aminoaniline, aniline, (o-, p-)chloroaniline, (2,5-,3,4-)chloromethylaniline, N-monomethylaniline,4,4′-diaminodiphenylmethane, N-phenyl-(α-, β-)naphthylamine,methylbenztriazole, 1,2,3-triazine, 1,2,4-triazine, 1,3,5-triazine,1,2,3-benztriazine, imidazole, 2-vinylpyridine, indole, quinoline,reaction products of monoethanolamine with o-vanillin, polyvinylalcohol, catechol, hydroquinone, resorcin, polyethyleneimine, disodiumethylenediaminetetraacetate, polyvinylpyrrolidone and the like.

Also, other leveling agents that can be effectively used includegelatin, polypeptone, N-(3-hydroxybutylidene)-p-sulfanilic acid,N-butylidenesulfanilic acid, N-cinnamoylidenesulfanilic acid,2,4-diamino-6-(2′-methylimidazolyl (1′))ethyl-1,3,5-triazine,2,4-diamino-6-(2′-ethyl-4-methylimidazolyl(1′))ethyl-1,3,5-triazine,2,4-diamino-6-(2′-undecylimidazolyl(1′))ethyl-1,3,5-triazine, phenylsalicylate, or benzothiazoles. Examples of the benzothiazoles includebenzothiazole, 2-methylbenzothiazole, 2-mercaptobenzothiazole,2-(methylmercapto)benzothiazole, 2-aminobenzothiazole,2-amino-6-methoxybenzothiazole, 2-methyl-5-chlorobenzothiazole,2-hydroxybenzothiazole, 2-amino-6-methylbenzothiazole,2-chlorobenzothiazole, 2,5-dimethylbenzothiazole,6-nitro-2-mercaptobenzothiazole, 5-hydroxy-2-methylbenzothiazole,2-benzothiazolethioacetic acid and the like.

(C. Vacuum Heating Treatment)

Vacuum heating of tin with the increased carbon concentration as neededsignificantly reduces the oxygen concentration and the sulfurconcentration in tin. When the tin is vacuum-heated, oxygen forming freeoxygen or an oxide in the tin reacts with carbon to form carbon monoxideor carbon dioxide which will be removed by volatilization. Further, bythis operation, the sulfur content in the tin is also significantlyreduced in the form of a compound such as elemental sulfur or sulfuroxide.

As described above, when elemental carbon is added to theelectrolytically refined tin, the elemental carbon can be added duringvacuum heating. Specifically, powdered elemental carbon is added to theelectrolytically refined tin in a vacuum melting state. The elementalcarbon includes, but not particularly limited to, graphite, carbonnanotubes, carbon fibers, fullerene, diamond and the like. The elementalcarbon may preferably have an average particle size of from 1 to 1000μm, and more preferably from 30 to 500 μm, and still more preferablyfrom 50 to 200 μm, in terms of dispersion and reactivity. In the presentinvention, the average particle diameter of the elemental carbon isdefined as D50 by volume, when measuring the particle size distributionby a laser diffraction scattering method.

In order to significantly reduce the oxygen concentration in tin, thetemperature, the degree of vacuum and the retention time of the vacuumheating treatment are important parameters. The temperature during thevacuum heating treatment may be preferably 400° C. or higher, and morepreferably 600° C. or higher, and even more preferably 800° C. orhigher, in terms of accelerating the reaction of oxygen with carbon, andfurther the reaction of oxygen with sulfur. However, although thetemperature during the vacuum heating treatment has a relation with thedegree of vacuum, tin will evaporate if the temperature is too high.Therefore, the temperature may be preferably 1200° C. or lower, and morepreferably 1100° C. or lower, and more preferably 1000° C. or lower. Thedegree of vacuum during the vacuum heating treatment may be preferablyhigher than 1×10⁻³ Pa (absolute pressure), and more preferably higherthan 1×10⁻⁴ Pa, and even more preferably higher than 1×10⁻⁵ Pa, in termsof effectively removing carbon monoxide or carbon dioxide produced bythe reaction of oxygen with carbon, free oxygen present in the tin, andfurther sulfur fraction such as elemental sulfur and sulfur oxide fromthe tin. However, the degree of vacuum during the vacuum heatingtreatment of higher than 1×10⁻⁵ Pa may not be desirable, in terms ofeconomic efficiency including facility.

The duration of the vacuum heating treatment may be a time required forsufficiently decrease the oxygen concentration and the sulfurconcentration in the tin. For example, the duration may be, for example,from 1 to 30 hours. The duration may be preferably 3 hours or more, andmore preferably 5 hours or more, in order to reduce the oxygenconcentration and the sulfur concentration, whereas the duration may bepreferably 25 hours or less, and more preferably 20 hours or less, interms of production efficiency.

(D. High Purity Tin)

In one embodiment of the high purity tin according to the presentinvention, the oxygen content as measured by elemental analysis usingDynamic-SIMS can be less than 10 ppb by mass.

In one embodiment of the high purity tin according to the presentinvention, the sulfur content as measured by elemental analysis usingGD-MS can be less than 0.1 ppm by mass, and preferably less than 0.05ppm by mass, and more preferably less than 0.01 ppm by mass.

In one embodiment of the high purity tin according to the presentinvention, the carbon content as measured by elemental analysis usingDynamic-SIMS can be less than 10 ppb by mass, and preferably less than 5ppb by mass.

EXAMPLES

Hereinafter, although Examples and Comparative Examples will beillustrated, these are provided for better understanding of the presentinvention, and the present invention is not intended to be limited dueto the Examples and Comparative Examples.

Example 1

Commercially available massive tin having a purity of 4N (99.99% bymass, excluding carbon, nitrogen, oxygen and hydrogen) was prepared.Analysis of the tin by the nondispersive infrared absorption methodshowed that the carbon content was less than 1 ppm by mass and theoxygen content was 20 ppm by mass. Analysis of the tin by the GD-MSmethod showed that the sulfur content was 1 ppm by mass. The tin as araw material was leached with 9N (normality) sulfuric acid at 80° C. for20 hours to prepare a sulfuric acid-acidic tin sulfate solution (a tinsulfate concentration of 80 g/L). A part of the sulfuric acid-acidic tinsulfate solution was removed, to which polyoxyethylene nonyl phenylether was added at a concentration of 8 g/L. Electrolysis was thencarried out at an electrolytic solution temperature of 25° C. and at acurrent density of 1 A/dm² to electrodeposit smooth tin on a surface ofa plate titanium cathode.

The resulting electrodeposited tin was heated to 300° C. in theatmosphere to obtain rectangular column shaped cast tin. Analysis of theatmospheric cast tin by the nondispersive infrared absorption methodshowed that the carbon content was 20 ppm by mass and the oxygen contentwas 10 ppm by mass. The sulfur content was 1 ppm by mass as measured bythe GD-MS method. The atmospheric cast tin was placed in a furnace in avacuum atmosphere and maintained at 800° C. under 1×10⁻⁴ Pa for 15 hoursfor vacuum casting. The refined tin thus obtained was measured byDynamic-SIMS, demonstrating that the oxygen content was less than 10 ppbby mass and the carbon content was less than 5 ppb by mass. The GD-MSdemonstrated that the sulfur content was less than 0.01 ppm by mass.Further, analysis of impurity elements other than oxygen, carbon andsulfur by the GD-MS method demonstrated that all of 73 elements otherthan carbon, nitrogen, oxygen and hydrogen were less than the detectionlimit. Therefore, it was confirmed that the purity of the resultingrefined tin was 6N (99.9999% by mass) or more.

Examples 2 to 4, Comparative Examples 1 to 3

The atmospheric cast tin of Example 1 was subjected to vacuum castingunder various conditions as shown in Table 1-1 and Table 2. The oxygenand carbon contents in the refined tin after the treatment were analyzedby the Dynamic-SIMS method and the sulfur content was analyzed by theGD-MS method. The results are shown in Tables 1-1 and 2. For Examples 2to 4, analysis of impurity elements other than oxygen, carbon and sulfurby the GD-MS method demonstrated that all of 73 elements other thancarbon, nitrogen, oxygen and hydrogen were less than the detectionlimit. Therefore, it was confirmed that each purity of the refined tinof Examples 2 to 4 was 6N (99.9999% by mass) or more. ComparativeExample 1 had the lower degree of vacuum, Comparative Example 2 had thelower maintaining temperature, and Comparative Example 2 had the shorterretention time, so that in each of these comparative examples, thedesirable decrease in the oxygen content could not be achieved.

Example 5

Electrolysis was carried out at an electrolytic solution temperature of27° C. and a current density of 3 A/dm² without adding any additive tothe sulfuric acid-acidic tin sulfate solution in Example 1 toelectrodeposit needle-like tin on a surface of a plate carbon cathode.The resulting electrodeposited tin was heated to 300° C. in theatmosphere to obtain cast tin. Analysis of the atmospheric cast tin bythe nondispersive infrared absorption method showed that the carboncontent was less than 1 ppm by mass, the oxygen content was 20 ppm bymass, and the sulfur content was 1.3 ppm by mass. To the atmosphericcast tin was added high purity graphite powder having an averageparticle diameter (D50) of 150 μm such that the carbon atom was at aratio of 50 mole per one mole of oxygen atom in the atmospheric casttin. It was placed in a vacuum atmosphere furnace, maintained at atemperature of 800° C. under 1×10⁻⁴ Pa for 15 hours for vacuum castingand refining. The refined tin thus obtained was measured byDynamic-SIMS, demonstrating that the oxygen content was less than 10 ppbby mass, and the carbon content was less than 5 ppb by mass. The GD-MSdemonstrated that the sulfur content was less than 0.01 ppm by mass.Further, analysis of impurity elements other than oxygen, carbon andsulfur by the GD-MS method demonstrated that all of 73 elements otherthan carbon, nitrogen, oxygen and hydrogen were less than the detectionlimit. Therefore, it was confirmed that the refined tin of Example 5 hada purity of 6N (99.9999% by mass) or more.

Examples 6 to 8, Comparative Examples 4 to 5

The atmospheric casted tin of Example 5 was subjected to vacuum castingunder various conditions as shown in Table 1-2 and Table 2. The oxygenand carbon contents in the refined tin after the treatment were analyzedby the Dynamic-SIMS method, and the sulfur content was analyzed byGD-MS. The results are shown in Table 1-2 and Table 2. For Examples 6 to8, analysis of impurity elements other than oxygen, carbon and sulfur bythe GD-MS method demonstrated that all of the impurities other thancarbon, nitrogen, oxygen and hydrogen were less than the detectionlimit. Therefore, it was confirmed that each purity of the refined tinof Examples 6 to 8 was 6N (99.9999% by mass) or more. ComparativeExample 4 had the decreased addition amount of elemental carbon, andComparative Example 5 did not add elemental carbon, so that in each ofthese comparative examples, the desirable decrease in the oxygen contentcould not be achieved.

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Addition Amount of 8 88 8 Leveling Agent (g/L) Oxygen Content 10 10 10 10 Before VacuumHeating Treatment (ppm by mass) Sulfur Content 1 1 1 1 Before VacuumHeating Treatment (ppm by mass) Carbon Content 20 20 20 20 Before VacuumHeating Treatment (ppm by mass) Addition Amount of None None None NoneElemental Carbon mol/mol - oxygen Degree of Vacuum (Pa) 1 × 10⁻⁴ 1 ×10⁻⁴ 1 × 10⁻⁴ 1 × 10⁻⁵ Maintaining Temperature 800 900 600 600 (° C.)Retention time (h) 15 10 20 15 Oxygen Content <10 <10 <10 <10 (ppb bymass) Sulfur Content <0.01 <0.01 <0.01 <0.01 (ppm by mass) CarbonContent <5 <5 <5 <5 (ppb by mass)

TABLE 1-2 Example 5 Example 6 Example 7 Example 8 Addition Amount ofNone None None None Leveling Agent (g/L) Oxygen Content 20 20 20 20Before Vacuum Heating Treatment (ppm by mass) Sulfur Content 1.3 1.3 1.31.3 Before Vacuum Heating Treatment (ppm by mass) Carbon Content <1 <1<1 <1 Before Vacuum Heating Treatment (ppm by mass) Addition Amount of50 70 20 30 Elemental Carbon mol/mol - oxygen Degree of Vacuum (Pa) 1 ×10⁻⁴ 1 × 10⁻⁴ 1 × 10⁻⁵ 1 × 10⁻⁵ Maintaining Temperature 800 800 900 900(° C.) Retention time (h) 15 20 15 20 Oxygen Content <10 <10 <10 <10(ppb by mass) Sulfur Content <0.01 <0.01 <0.01 <0.01 (ppm by mass)Carbon Content <5 <5 <5 <5 (ppb by mass)

TABLE 2 Comparative Comparative Comparative Comparative ComparativeExample 1 Example 2 Example 3 Example 4 Example 5 Addition Amount of 8 88 None None Leveling Agent (g/L) Oxygen Content 10 10 10 20 20 BeforeVacuum Heating Treatment (ppm by mass) Sulfur Content 1 1 1   1.3   1.3Before Vacuum Heating Treatment (ppm by mass) Carbon Content 20 20 20 <1<1 Before Vacuum Heating Treatment (ppm by mass) Addition Amount of NoneNone None   0.5   0.0 Elemental Carbon mol/mol - oxygen Degree of Vacuum1 × 10⁻² 1 × 10⁻⁴ 1 × 10⁻⁴ 1 × 10⁻⁴ 1 × 10⁻⁵ (Pa) Maintaining 800 300800 800  800  Temperature (° C.) Retention time (h) 15 20 0.5 15 20Oxygen Content 1200 1500 1500 10000~  10000~  (ppb by mass) SulfurContent 0.7 0.9 1.0   0.5   0.8 (ppm by mass) Carbon Content <5 <5 <5 <5<5 (ppb by mass)

The refined tin obtained by vacuum casting was then melted and cast toproduce a high purity tin ingot. The high purity tin ingot produced wasused as a raw material of a tin target material used for generating EUVlight in a lithographic EUV light source. As a result, the tin targetdid not result in clogging of a nozzle of a droplet generator when thetin target was heated and melted to produce droplets with a smalldiameter of about 25 μm.

The above results showed that the high purity tin ingots each havingextremely low oxygen, carbon and sulfur contents, produced in Examples 1to 8, had little deposition of silicon and carbon due to dust in theatmosphere and was barely influenced by atmospheric oxidation, and couldsufficiently suppress contamination of impurities that would causeparticles.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a highpurity tin having an extremely low content of oxygen in the tin, whichoxygen has been conventionally difficult to be removed. According to thepresent invention, it is also possible to obtain a high purity tinhaving the lower sulfur content. The high purity tin from which oxygenand sulfur have been removed can prevent nozzle clogging due to oxidesand sulfides and enables to obtain droplet tin with a stable shape.Therefore, such a high purity tin is useful as a tin target material inany form, used in the EUV light source.

What is claimed is:
 1. A high purity tin having a tin purity of (99.999%by mass or more, excluding carbon, nitrogen, oxygen and hydrogen,wherein the high purity tin has an oxygen content of less than 10 ppb bymass, as measured by elemental analysis using Dynamic-SIMS.
 2. The highpurity tin according to claim 1, wherein the high purity tin has asulfur content of less than 0.1 ppm by mass, as measured by elementalanalysis using GD-MS.
 3. The high purity tin according to claim 2,wherein the sulfur content is less than 0.01 ppm by mass, as measured byelemental analysis using GD-MS.
 4. The high purity tin according toclaim 1, wherein the high purity tin has a carbon content of less than10 ppb by mass, as measured by elemental analysis using Dynamic-SIMS. 5.A tin target material comprising the high purity tin according to claim1, used for generating EUV light in a lithographic EUV light source.